Recently, with the diversification of the objectives of electrography, various toners or developers have been studied and developed depending on the respective objectives. Indeed, the principal object of such toner is to form a precise image, but various factors as, for example, electro-charging properties, carrying properties, fixing characteristics, preservation properties and the like, should be possessed by the toner particles themselves.
In the fixing of toner images, various methods have been proposed and among the known methods, the most popular one is the heat roll-fixing method. However, in the conventional heat roll-fixing method, a part of the image-forming toner particles is transferred on the surface of heat-roll at the time of offset development, i.e. at the fixing stage, which is again moved to the subsequent copying paper, resulting in the contamination thereof. Therefore, anti-offset properties, that is the hard transfer of toner particles to the heating roll surface, is strongly desired. In order to prevent the undesired offset phenomenon at the heating-roll-fixing stage, the recent toner particles contain releasing agents such as polyolefins (e.g. polypropylene, polyethylene and the like), paraffin wax and the like.
Attempts have been made to increase the amount of such releasing agent for better anti-offset property, and however, there often arises new question of melt-adhesion of toner particles during the preparation thereof.
In another approach, crosslinked resin or a mixture of different resins each having different molecular weight distribution had been used as a binder resin. However, in such methods there were problems that a much higher temperature should be used for the fixing purpose. (see Japanese Patent Publication No.23354/76, Japanese Patent Publication (unexamined) 217357/88).
In another method, (Japanese Patent Publication (unexamined) 210368/86), binder resin and colorant are dispersed on the surface of the respective spherical particles by using Henshel mixer, Super mixer or the like and they are fixed there upon by applying a heating energy at a temperature which is lower than the softening point of said spherical particle and higher than the softening point of said binder resin. However, in this method, besides the problem of limited materials, there are problems that technically, it is very difficult to obviate the formation of fused mass of spherical particles at the heating stage (usually at 110.degree.-140.degree. C., for 10 minutes) and in some cases, there arises the thermal degradation of the employed materials. There are also such proposals that releasing microparticles are applied on the toner particles (see Japanese Patent Publication (unexamined) 244053/88 and charge controlling microparticles are applied on the toner particles (Japanese Patent Publication (unexamined) 244056/88).
The above procedures are, indeed, effective in giving a fairly good solution for each separate problem, but do not fully satisfy every aspect of the requirements for ideal toners. Furthermore, the heretofore proposed dry toners cannot be used in the latest high speed copying machines.
It is, therefore, an object of the invention to provide a dry toner for a high speed copying use, which is excellent in anti-offset properties, can be easily adjusted or controlled in electrification and has a comparatively low fixing temperature. An additional object of the invention is to provide a dry toner being specifically useful in a high speed copying, and having excellent powder-coating properties as electrification, developing amount, cleaning property, carrying property and the like which are desired for the image-improvement in respects of ultra-fine-live-reproduction, texture, dot reproduction, toning, resolving power and the like. Further object of the present invention is to provide a dry toner having spherical or quasi-spherical from and being excellent in application properties.